Rules for Tools/popuprhythm()
struct Rhythm *popuprhythm(struct Rhythm *originalrhythm)
`struct Rhythm *originalrhythm'
`popuprhythm()' opens a scrolling pop-up menu with all of the
system rhythms and lets the user choose a new rhythm.
`popuprhythm()' returns a pointer to the selected rhythm. If the
user doesn't select any rhythm, `popuprhythm()' returns the
previous rhythm, originalrhythm. If there was no previous rhythm
(originalrhythm == 0), and the user does not select a new one
(`popuprhythm()' returns 0), be sure to handle this gracefully.
See popupscale(), popupchord().